Fluid combination apparatus

ABSTRACT

A first unidirectional fluid conducting device having a constant pressure output conducts fluid from a first fluid source to one input of a selector valve and a second unidirectional fluid conducting device having a constant pressure output conducts fluid from a second fluid source to another input of the selector valve. The selector valve is responsive to the pressure of the second fluid source for connecting the output of the first unidirectional fluid conducting device through the selector valve to the output thereof when the pressure of the second fluid source falls below a predetermined level. A shunt valve is connected between the inputs to the selector valve, the shunt valve being responsive to the pressure of the first fluid source for connecting together the inputs of the selector valve when the pressure of the first fluid source falls below a predetermined level. Therefore, fluid conduction to the output of the selector valve is assured from either one fluid source or the other. In addition, the selector valve is selectively operated to conduct fluid from either the first or the second fluid source in amounts as desired to obtain a predetermined combination of the first and second fluids in a fluid reservoir attached to the selector valve output. The input to the reservoir contains a Venturi mixing device to uniformly combine incoming fluids with the fluids within the reservoir. A portion of the mixed fluid is removed from the reservoir and conducted to a selective fluid detector which is sensitized to one of the fluids in the mixture. Control circuitry is attached to the detector to provide a signal to selectively operate the selector valve to conduct the required amounts of fluid from the first and second sources whereby the desired fluid combination and mixture is obtained.

United States Patent [191 Williams FLUID COMBINATION APPARATUS [76]Inventor: Andrew Sinclair Williams, Brea,

Calif.

[22] Filed: Dec. 4, 1972 [21] Appl. No.: 311,493

OTHER PUBLICATIONS Closed Cycle Breathing System Extends Diving Time,Design News, July 21, 1969, pages 14 & 15.

Primary Examiner-Robert G. Nilson Assistant Examiner-Edward LookAttorneyRaymond L. Madsen [57] ABSTRACT A first unidirectional fluidconducting device having a constant pressure output conducts fluid froma first fluid source to one input of a selector valve and a secondunidirectional fluid conducting device having a Nov. 13, 1973 constantpressure output conducts fluid from a second fluid source to anotherinput of the selector valve. The selector valve is responsive to thepressure of the second fluid source for connecting the output of thefirst unidirectional fluid conducting device through the selector valveto the output thereof when the pressure of the second fluid source fallsbelow a predetermined level. A shunt valve is connected between theinputs to the selector valve, the shunt valve being responsive to thepressure of the first fluid source for connecting together the inputs ofthe selector valve when the pressure of the first fluid source fallsbelow a predetermined leveli Therefora fluid conduction to the output ofthe selector valve is assured from either one fluid source or the other.In addition, the selector valve is selectively operated to conduct fluidfrom either the first or the second fluid source in amounts as desiredto obtain a predetermined combination of the first and second fluids ina fluid reservoir attached to the selector valve output. The input tothe reservoir contains a Venturi mixing device to uniformly combineincoming fluids with the fluids within the reservoir. A portion of themixed fluid is removed from the reservoir and conducted to a selectivefluid detector which is sensitized to one of the fluids in the mixture.Control circuitry is attached to the detector to provide a signal toselectively operate the selector valve to conduct the required amountsof fluid from the first I and second sources whereby the desired fluidcombination and mixture is obtained.

15 Claims, 3 Drawing Figures FLUID COMBINATION APPARATUS The presentinvention relates to combining fluids in predetermined proportions andmore particularly to the mixing of gases under pressure in accuratelyknown concentrations for use with medical respirators and relatedequipment.

In the field of fluid combining and mixing, it has been the generalpractice to employ manually adjustable proportional controllers andautomatically controlled systems of valves and conduits to control theconcentration of a given fluid in a mixture of fluids. Although suchdevices have served the purpose, they have'not proved entirelysatisfactory under all conditions of service for the reason thatconsiderable difficulty has been experienced in obtaining a requiredfluid flow in the absence or failure of one of the input sources offluid being mixed and difficulties encountered in accurately controllingthe concentration of a particular fluid in the mixture.

When oxygen is mixed with air to provide an air enriched mixture for usewith respirators, it has been the general practice to utilize pressureoperated valves and mechanical controllers to control the mixture. Thesevalves and controllers have not been entirely satisfactory for thereason that the oxygen concentration is often in excess of that desiredwith resulting irreversible tissue damage to the patient. Further, ifthe equipment fails, it is common to lose both the supply of oxygen andair resulting in loss of a breathing environment to the patient.

Those concerned with the development of life support systems andrespirators have long recognized the need for a fail safe automaticallycontrolled system in which the subject is assured of a breathingenvironment under conditions of partial or total failure of thecontrolling apparatus. The present invention fulfills this need. 7

One of the most critical problems confronting designers of breathingsystems and medical respirators has been the provision ofaccurateautomatic control of adjustably mixed breathing gases. Thisproblem is overcome by the present invention.

Prior fluid blending systems have utilized ratio controllers which areresponsive to flow controllers for maintaining a predetermined ratiobetween the flows of streams of fluids which are to be combined, asillustrated in U. S. Pat. No. 3,124,148. Here, an analyzer is used tocontinuously measure a property of the blend stream, such as refractiveindex, and to control the ratio of the streams blended in response tothis analysis. THe analyzer operates to reset the ratio controller asmay be necessary to maintain the composition of the blended streamconstant at a predetermined value. Further, the streams are delivered toa vessel wherein mechanical propellers agitate the blend in order toobtain uniform mixing. However, in this type of system, loss of one ofthe fluid streams results in the ratio controller shutting off theremaining stream and opening the valve full to the failed stream whichin turn results in the failure of any fluid reaching the mixing chamberor reservoir. The present invention overcomes this difiiculty.

A gas analyzer controlled gas mixing apparatus is illustrated in U. S.Pat. No. 3,465,753 for the controlled application of an anesthesia to apatient. An ultraviolet analyzer controls the actuation of valves and apump for the accurate introduction of halophane into the patientsbreathing supply. However, should the supply of oxygen fail or thecontrol apparatus fail, there is no fail-safe operation of the apparatusto insure that the patient does not obtain an excessive amount ofanesthesia or the lack of a life supporting breathing environment. Thepresent invention overcomes this problem.

An artificial atmosphere system as described in U. S. Pat. No. 3,215,057in which pressure sensors are used to indicate the partial pressure ofoxygen and total pressure of a gas mixture delivered to a life supportsystem. The pressure sensors are used to control valves at theinletsfrom oxygen and nitrogen sources. There are no fail-safe featuresprovided to guarantee a minimal life supporting atmosphere nor are thereany mixing devices to provide a reliable mixture of the combined gases.The present invention overcomes these deficiencies.

In U. S. Pat. No. 3,256,900, there is illustrated an electrochemicalsensing unit in a control system to provide a relatively simple andreliable system for regulating the concentration of a depolarizer gas ina given gas mixture. The sensing unit utilized contains two cells whichgenerate a voltage varying with the oxygen partial pressure in the gasto which the cathode of each cell is exposed. The cells are connectedtogether electrically opposing each other andthe potential voltagedifferential is measured. It is necessary that the cells be operatedunder a pulse current load to overcome uncontrollable variations ofinternal resistance of each cell as well as those due to temperaturechanges of the conductivity of cell contacts and leads. The presentinvention overcomes these problems.

The general purpose of this invention is to provide a fluid mixing andcontrol system which embraces all the advantages of similarly employedfluid mixing and control systems and possesses none of theaforedescribed disadvantages. To attain this, the present inventioncontemplates a unique arrangement of conduits and pressure sensitivevalves whereby failureof the system to provide at least one of the inputsources of fluid to an output mixing reservoir is avoided. In addition,the present invention utilizes a unique mixing device in the mixingreservoir in order to avoid unmixed pockets of fluid and erroneousindications of mixture therefrom. Furthermore, the present inventioncontemplates a unique control system in which a polarographic selectivedetector is employed whereby uncontrolled variations of internalresistance are'avoided.

An object of the present invention is the provision of the fail-safemixing of fluids by assuring the connection of the system to one of theremaining sources of fluid should the other sources fail.

Another object is to provide the thorough mixing of combined fluids in amixing reservoir.

Yet another object of the present invention is the provision of a fluidselection process whereby one or another of two fluid inputs arealternately selected automatically to provide a predetermined mixture ofthe fluids in a mixing reservoir.

A still further object is to provide the automatic mixing of oxygen andair in a predetermined accurate concentration independent of linepressure and flow variations.

A still further object of the invention is the provision of theautomatic mixing of oxygen and air with the fail-safe provision ofsupplying pure oxygen should the air supply fail or air should theoxygen supply fail.

Still yet another object of the present invention is the provision ofautomatic mixing of oxygen in air in preset proportions controlled by apolarographic sensor for use in a respirator for medical application.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings in which like referencenumerals refer to like parts throughout the figures thereof and wherein:

FIG. 1 shows a mechanical block diagram of a preferred embodiment of theinvention;

FIG. 2 illustrates a schematic diagram of the electronic control portionfor the embodiment illustrated in FIG. 1; and

FIGS. 3 (a) and (b) illustrate a plan view and section view,respectively, of a preferred embodiment of a Venturi mixing deviceutilized in the mixing tank of FIG. 1.

Turning now to FIG. 1, conduit 7 connects a source of fluid connected toIN 1 to check valve which in turn is connected to regulator valve 13having a constant pressure output connected to conduit 17. Similarly,conduit 11 connects a second source of fluid connected to IN 2 to checkvalve 9 which in turn is connected to regulator valve 15 having aconstant pressure output connected to conduit 19. Conduits l7 and 19 areconnected through valve 21 which is normally closed and which is openedby pressure mechanism 23 attached thereto and further connected to pilotconduit 25 which is connected to conduit 7 when the pressure in conduit7 drops below a predetermined level. Selector valve 27 has one input 29connected to conduit 17 and another input 31 connected to conduit 19 andfurther has mechanism 35 connected thereto which in turn is connected toconduit 11 through pilot conduit 37. Mechanism 35 has a solenoid portionconnected to a pair of electrically conducting leads through which thesolenoid is operated to connect input 31 to output 33 of selector valve27. Mechanism 35 further has a pressure operated section which isactuated when the pressure in conduit 11 drops below a predeterminedlevel to direct selector valve 27 to connect input 29 to output 33thereof. When the solenoid portion of mechanism 35 is not operated,input 29 is normally connected to output 33 of connector valve 27.Output 33 of selector valve 27 is connected to Venturi mixing device 41in mixing tank or reservoir 39. Reservoir 39 has one output connected toconduit 43 which is also connected to a pressure indicator 45. Conduit43 conducts the mixture from reservoir 39 for distribution. Anotheroutput of rservoir 39 is connected to reducing valve or small orifice 47which in turn is connected to an input of selector valve 49. Selectorvalve 49 has another input thereto connected to reducing valve or smallorifice 51 which in turn is connected by conduit a to conduit 17.Another input to selector valve 49 connected to reducing valve or smallorifice 53 which in turn is connected through conduit b to conduit 19.Another input to selector valve 49 is connected an external source offluid through input 35. Selector valve 49 connects any one of theaforementioned inputs through an output thereof to detector 57 which inturn is connected to an exhaust port. Detector 57 has a pair ofelectrically conducting leads x across which can be measured anelectrical parameter proportional to the concentration of a specificfluid in the mixture of fluids directed thereto from reservoir 39.

Referring now to FIG. 2, bias supply 59 is connected between circuitground and one of the pair electrical leads x of detector 57 the otherlead being connected to inverting input 61 of operational amplifier 63,noninverting input 65 of amplifier 63 being connected through resistor67 to circuit ground. Resistor 69 is connected from inverting input 61to output 71 of operational amplifier 63, output 71 being furtherconnected to meter 73 which in turn is connected to circuit ground.Output 71 of operational amplifier 63 is further connected throughresistor 75 to'inverting input 77 of operational amplifier 79,non-inverting input 81 of amplifier 79 being connected through resistor83 to circuit ground. Non-inverting input 81 is further connectedthrough resistor 85 to adjustable arm 87 of potentiometer 89.Potentiometer 89 isconnected between a negative source of referencevoltage (-REF. V) and circuit ground. Inverting input 77 of amplifier 79is further connected through resistor 91 to output 93 of amplifier 79,which in turn is connected to stabilizing network 95 and to comparator123.

Stabilizing network 95 has capacitor 97 connected in parallel withresistor 99, the parallel combination being connected between output 93of amplifier 71 and one end of resistor 101, the other end of resistor101 being connected to circuit ground. The junction of capacitor 97,resistor 99, and resistor 101 is further connected to valve controlswitch 103 WHICH includes operational amplifier 107, transistor 115 andsolenoid 117. Noninverting input 105 of amplifier 107 is connected tothe junction of capacitor 97, resistor 99 and resistor 101, invertinginput 109 of amplifier 107 being connected to circuit ground. Output 111of amplifier 107 is connected through current limiting resistor 113 tothe base of NPN transistor 115, the emitter of which is connected tocircuit ground. The collector of transistor 115 is connected throughsolenoid 117 to a source of plus voltage (+V). Fixed contact 119 ofsolenoid 117 is connected to one of the pair of electrically conductingleads Y connected to the solenoid of mechanism 35, the other of the pairof leads being connected to one side of a source of l 15 V 60 Hz. Theother side of the 115 V 60 Hz is connected to moving contact 121 ofsolenoid 117.

Comparator 123 contains operational amplifiers 121 and 131,non-inverting input 125 of amplifier 129 being connected to invertinginput 127 of amplifier 131 and further connected to output 93 ofamplifier 79. Inverting input 133 of amplifier 129 is connected to aplus reference voltage (+REF. V) and non-inverting 135 of amplifier 131is connected to a negative reference voltage (REF. V); Output 137 ofamplifier 129 is connected to output 139 of amplifier 131 and is furtherconnected through current limiting resistor 141 to the base of NPNtransistor 143 having the emitter thereof connected to circuit ground.Collector of transistor 143 is connected to the cathode of diode 145 theanode of which is connected to one lead of lamp 151 and to lead 147 ofbuzzer 149. The other lead of lamp 151 is connected through source tocircuit ground; The junction of source 155 and lamp 151 is furtherconnected through switch 153 to the other lead of buzzer 149.

Turning now to FIG. 3(a), there is shown a side plan view of a preferredembodiment of mixing device 41 of FIG. 1. The device shown iscylindrical in nature having a threaded section 157 upon which isthreaded nut 159 by which it is secured in reservoir 39 of FIG. 1 Asmooth cylindrical section 161 has holes 163 centrally located thereonaround its circumference.

Referring now to FIG. 3(b), there is illustrated a cross sectional viewof the preferred embodiment of FIG. 3(a). Interior to the cylindricalsection 161 is centrally located a coaxial cylindrical conduit sectionor nozzle 165 which is attached to threaded section 157 and having itsopen end terminating approximately in the vicinity of holes 163. Opening167 is coaxially located interior to and traverses threaded section 157and continues through the center of nozzle 165 into the interior ofcylindrical section 163.

Operation of the invention can best be described by turning to FIG. 1where two fluid inputs are respectively connected to IN 1 and IN 2. Thefluid connected to IN 1 is directed through check valve 5 and regulatorvalve 13, check valve 5 preventing undesirable backflow and regulatorvalve 13 providing fluid at constant pressure to conduit 17 and to inlet29 of selector valve 27. Similarly, the fluid connected to IN 2 flowsthrough conduit 11 through check valve 9 and regulator 15, check valve 9preventing undesirable backflow and regulator valve providing a constantpressure flow into conduit 19 equal to that in conduit 17 therebyoperating inlet 31 and inlet 29 of selector valve 27 at constant andequal pressures.

Selector valve 27 is of the solenoid operated pilot type, such asthree-way valve, model No. 125A-3-l l- 20 manufactured by Humphrey. Inthe event that fluid pressure at IN 2 fails, selector valve 27 willautomatically position itself to connect inlet 29 to outlet 33 toconduct gas from IN 1 to outlet 33. This is accomplished by sensing thepressure at IN 2 through pilot conduit 37 by mechanism 35 wherein theloss of pressure in pilot conduit 37 causes mechanism 35 to actuateselector valve 27 to switch inlet 29 to outlet 33. In the event of powerfailure, selector valve 27 also assumes the position connecting inlet 29to outlet 33.

Shunt valve 21 is a presure operated normally open type valve and may beof the type such as the three-way valve used for selector valve 27 Thefluid pressure provided through pilot conduit to mechanism 23 preventsshunt valve 21 from opening and therefore, under normal conditions, withfluid pressure at IN 1, bypass valve 21 remains closed. If pressurefails at IN 1, valve 21 opens allowing the fluid from IN 2 to flowthrough valve 21 and through selector valve 27 to outlet 33 regardlessof the position of selector valve 27 In this way, fluid is conducted tooutlet 33 under all fault conditions except loss of all input fluidsources.

The fluid from outlet 33 of selector valve 27 enters Venturi mixingdevice 41 wherein the flow of fluid creates a Venturi pressure drop thatforces the fluid from the interior and walls of the tank into theflowing stream of the Venturi mixing device to move and mix by narrowbore pipe to mixing tank or reservoir 39 and to sample selector valve49, which selects one of four low-flow rate inputs. Small orifice 47substantially provides fluid at constant flow to selector valve 49. Theoutput of selector valve 49 is directed into detector 57.

When gases such as oxygen and air are being mixed, detector 57 may be apolarographic detector similar to the type described in U. S. Pat. No.2,9l3.,386 wherein the concentration of oxygen in air is detected. Inthis application, the detector acts as a variable resistance, theresistance being substantially infinite with no oxygen concentrationpresent in the mixture and decreases inversely proportional to the Theof oxygen in the mixture. THe pair of leads x provide connection tomeasure this electrical characteristic.

It should be noted that the bore diameters of the conduit lines frommixing tank or reservoir 39 to the detector are made small to minimizefluid transit time. The other sample inputs to selector valve 49 are forcalibration and remote control of the fluid mixture. When the selectorvalve input is connected to reducing valve or small orifice 51, a sampleof fluid from IN 1 through check valve 5 and regulator valve 13 issupplied through reducing valve 51 substantially at constant flow andthrough selector valve 49 to detector 57. Similarly, when the input ofselector valve 49 is connected to reducing valve or small orifice 53, asample of fluid connected to IN 2 is supplied substantially at constantflow to detector 57. Since the input pressures to reducing valve orsmall orifice 47, 51 and 53 are substantially the same and the reducingvalves produce substantially the same flow rate, the detectorexperiences a constant and substantially equal fluid flow from any oneof the positions. When the input to selector valve 49 is connected toinput 55, an external sample can be analyzed from some remote part ofany system to which the mixing apparatus is connected.

Although detector 57 has been described as a polarographic type,sensitive to oxygen when oxygen was one of the mixed fluids, the basicrequirement is that the detector must be of a type sensitive to one ofthe two input fluids applied to IN 1 and IN 2 of the system. Forexample, when the fluid applied to IN 1 is oxygen and the gas applied toIN 2 is air or nitrogen, an oxygen sensor such as that described abovewould be employed. When mixing, for example, nitrogen, and carbondioxide, either a carbon dioxide or nitrogen sensor could be usedprovided that the sensor chosen can operate independently within theenvironment of the other gas. In any event, the detector should producean electrical signal or have an electrical characteristic which variesas some function of the concentration of one of the fluids in themixture and which may be applied to an electronic control circuit.

Referring now to FIG. 2, there is set forth a typical circuit diagram ofa control circuit that can be used with an oxygen polarographic detectoror sensor described hereinbefore. It should be noted that otherdetectors are contemplated and input modifications of the amplifiercircuit are anticipated depending on the type of sensor utilized. Wherethe sensors are sensitive to temperature variations, it will benecessary to provide compensation for these temperature variations.These techniques are well known to one skilled in the design ofelectronic circuitry. Bias supply 59 provides a bias to detector 57 suchthat a current flows through the detector inversely proportional to theresistance of the detector. The current flows through resistor 69 ofoperational amplifier 63 causing the voltage output of amplifier 63 tobecome negative with respect to circuit ground. Amplifier 63 can be ofthe integrated circuit type such as SN 72709L manufactured by TexasInstruments. Resistor 69 can be a thermistor to provide temperaturecompensation for the input sensor or detector.

Meter 73 is connected to the output of amplifier 63 to provide anindication of the percentage of the gas connected to In 1 in the mixturewithin mixture tank or reservoir 39. A comparison is now made betweenthis electrical analog of gas concentration and an adjustable referenceor set-point concentration provided by the voltage at adjustable arm 87of potetiometer mixture control 89. Amplifier 79, which may be of thesame type as amplifier 63, provides a current through resistor 75proportional to the difference in the mixture voltage across meter 73and the mixture control or set-point voltage at adjustable arm 87 ofpotentiometer 89. If the concentration of the gas at IN 1 provided tothe reservoir 39 is greater than the valve set by the mixture controlvoltage of potentiometer 89, the voltage at the output of amplifier 63will be lower than the reference voltage at adjustable arm 87 ofpotentiometer 89 causing a current to flow from the output of amplifier79 through resistor 91 and resistor 75 thereby producing a positiveerror signal at the output of amplifier 79 which is applied throughstabilizing network 95 to operational amplifier 107. Operationalamplifier 107 may be of the same type as amplifier 63 and 79. Thepositive error signal in turn causes the output of amplifier 107 to bepositive which in turn renders transistor 115 conductive therebyoperating solenoid 117 and closing swinging contact 121 to fixed contact119 thereof. Thus, the l V 60 Hz source is connected to the solenoid ofmechanism 35 causing selector valve 27 to be placed into the positionwhich connects input 31 to output 33, thereby providing fluid from IN 2to the reservoir 39 to reduce the relative concentration of the fluidreservoir 39 received from IN 1. Therefore, a close loop regulatorcircuit has been established to control the concentration of the fluidmixture in reservoir 39.

Stabilization network 95 provides a phase lead to compensate for thelagging time constants of the fluid transit times through the conduitsand the mixing tank as well as the time constant of the detector orsensor itself. The time constant of fluid transit can be minimized byusing short conduit runs and small bore diameter conduits consistentwith the requirements of adequate output flow. In its simplest form, thestabilization network can be a simple single time constant, phase leadcircuit illustrated by the parallel combination of capacitor 97 andresistor 99 connected in series to the input of operational amplifier107, the input being shunted by resistor 101 to circuit ground. Resistor99 may be typically ten times the value of resistor 101 and the timeconstant CR of the combination of resistor 99 capacitor 97 should be noless than the longest time constant of any portion of the loop, which isusually the sensor or detector. In typical operation, the stabilizingnetwork introduces an anticipatory signal to valve control switch 103which is dependent on the rate of change of the relative fluidconcentrations in reservoir 39.

An additional feature of the apparatus while not essential to the mixingfunction, provides visual and audible alarms when the fluid mixturefalls outside of a predetermined interval. Operational amplifiers 129and 131, which may be of the integrated circuit type such as ComparatorSN 7271 1L manufactured by Texas Instruments, are connected to provide adual level comparator 123 which delivers an output signal when eithertwo preset reference potentials (one positive on inverting input 133 ofamplifier 129 and one negative on noninverting input 135 of amplifier131) have been exceeded. Thus, when the potential at output 93 ofamplifier 79 exceeds the plus reference voltage or falls below thenegative reference voltage connected to comparator 123, the output ofcomparator 123 becomes positive and is applied through resistor 141 totransistor 143 rendering transistor 143 conductive which in turn tumsonlamp 151 through diode and when switch 153 is closed, also operatesbuzzer 149. Alternating supply 155 applies half cycles of an alternatingsignal to lamp 151 through diode 145. Diode 145 prevents conduction onthe reverse or negative cycles of alternating source 155. An alternatingsource 155 is utilized to enable the application of an alternatingcurrent buzzer 149 for more reliable audible alarm operation. The buzzermay typically be one manufactured by Potter Brumfield, Model No. BU24AC.Transistor 143 may be of the NPN type 2N5449 manufactured by TexasInstruments. Switch 153 may be opened to eliminate the audible alarmwhen desired. In a typical application where the fluids are gases andthe gas applied to IN 1 is oxygen and the gas applied to IN 2 is air,the preset reference voltages to comparator 123 would be set at $5percent oxygen error about the adjustable mixture set point. This meansthat the alarm lamp 151 and buzzer 149 are actuated whenever the oxygenconcentration deviates greater :5 percent from the mixture set pointdetermined by mixture control potentiometer 89.

Turning now to FIG. 3(b), there is illustrated a cross sectional view ofthe Venturi mixing device 41 in reservoir 39 of FIG. 1. Gas enteringopening or conduit 167 exits through inner cylindrical section or nozzle165, where the gas stream flow causes a reduced pressure inside cylindersection 161 adjacent holes 163, thereby forcing fluid outside cylindersection 161 through holes 163 to intermingle with the stream of fluidflowing from nozzle 165. By this action, a positive mixing phenomenon isestablished to keep the gases within reservoir 39 in constant motionthereby providing a thorough mixing process and an accurate and reliablecombining of the gases being mixed. This feature is desirable to avoidany erroneous indications in the mixing tank of the gas mixture which isdirected to detector 57. Pockets of unmixed gas may cause detector 57 toindicate the need for providing more or less of one gas or the otherwhere in fact the net concentration was that desired had the gases beenthoroughly mixed. It should be noted by reference to FIG. 1 thatselector valve 27 operates to either provide gas from one input or theother input and is not restricted to controlling the gas from a singleinput. Therefore, considerably faster changes in mixtures can beachieved and a wider control accomplished than in most prior artsystems. 7

It now should be apparent that the present invention provides anautomatic fluid mixing control system such as that which may be employedin conjunction with a' respirator and related equipment for providingair enriched in oxygen where accurately known mixtures of air and oxygenare required for medical purposes. The need for accurate control ofoxygen for medical applications is desirable in light of the toxiceffects and irreversible tissue changes caused by excessive oxygenexposure. The present invention employs automatic feedback control inconjunction with actual analysis of a fluid mixture. In this respect itis markedly superior to earlier devices which rely on manual flowadjustment of the individual gases or upon pneumatic proportioning. Thepresent invention is not sensitive or susceptible to variations in linepressure in output flow variations of the fluids. It is also possible toremote monitor the concentration on an external piece of equipmentconnected to the subject invention thereby compensating for leakage andother errors introduced by the connected equipment. When used with arespirator supplying a mixture of air and oxygen, accurate control anddisplay of oxygen concentration at the present patient intake isaccomplished.

Although particular components, etc., have been discussed in connectionwith a specific embodiment of fluid mixing apparatus constructed inaccordance with the teachings of the present invention, others may beutilized. Furthermore, it will be understood that although an exemplaryembodiment of the present invention has been disclosed and discussed,other applications and mechanical and electrical circuit arrangementsare possible and that the embodiments disclosed may be subjected tovarious changes, modifications and substitutions without necessarilydeparting from the spirit of the invention.

What is claimed is:

l. Fluid combining apparatus comprising:

first and second means, each having an input and an output forconducting fluid in one direction from said input to said output, meansfor maintaining the fluid at said output at substantially constantpressure, said inputs being adapted for connection to sources of fluid;

shunt means connected between the outputs of said first and secondmeans, said shunt means being responsive to the pressure of the fluid atsaid first means input for connecting together the outputs of said firstand second means when the pressure of the fluid at said first meansinput falls below a predetermined level; and Y selector means having twoinputs and an output for directing fluids from any one of said twoinputs of said selector means to said output of said selector means,said two outputs of being connected respectively to the outputs of saidfirst and second means, said selector means being responsive to thepressure of the fluid at said second means input for connecting saidoutput of said first means to said selector means output when thepressure of the fluid at said second means input falls below apredetermined level. I 2. The flu'id combining apparatus described inclaim 1 furthercomprising reservoir means having an input and at leastone output, said input being connected to said selector means output forreceiving and combining the fluids conducted by said selector means.

3. The fluid combining apparatus described in claim 2 further comprisingmixing means within said reservoir means for substantially interminglingand dispersing the fluids uniformly throughout the confines of saidreservoir means.

4. The fluid combining apparatus described in claim 3 further comprisingmeans for producing substantially constant fluid flow connected to saidat least one output of said reservoir means.

5. The fluid combining'apparatus described in claim 4 furthercomprising:

detector means connected to said means for producing constant fluidflow, said detector means generating an electrical signal in response tothe amount of fluids received from said first means in said reservoirmeans; and

electromechanical means attached to said selector means and saiddetector means, said electromechanical means being responsive to saiddetector means electrical signal for directing said selector means toconnect said second means output to said selector means output wherebythe fluid conducted by said second means is conducted through saidselector means into said reservoir means when the fluids received insaid reservoir means from said first means exceeds a predeterminedconcentration. a

6. The fluid combining apparatus described in claim 5 wherein saiddetector means comprises:

a fluid detector having a pair of electrical leads between which thereis a resistance inversely proportional to the concentration of thefluids received in said reservoir means from said first means; and

an electronic amplifier circuit connected to said pair of electricalleads, said amplifier having an output connected to saidelectromechanical means at which output said electrical signal isvgenerated when said fluid detector resistance falls below a preselectedmagnitude, said amplifier having means for adjusting said preselectedmagnitude whereby the fluid mixture is controlled.

7. The fluid combining apparatus described in claim 6 wherein the fluidsare gases and wherein said fluid detector comprises: 1

a polarographic cell having electrolyte in which an anode and cathodeare immersed, and a selective permeable barrier for separating saidelectrolyte from the gases being analyzed, said barrier being permeableto a specific gas whereby the gas enters tion of the fluid received insaid reservoir means from said first means falls outside of apredetermined interval.

10. The fluid combining apparatus described in claim 6 wherein saidelectronic amplifier comprises? a bias voltage supply having itspositive terminal connected to one of said pair of electrical leads ofsaid indicating the concentration of the fluid received in saidreservoir means from said first means;

a second operational amplifier having its inverting input coupled to theoutput of said first operational amplifier, said second operationalamplifier having its non-inverting input connected to an adjustable,source of reference voltage whereby the concen-' tration of the fluidreceived in said reservoir means from said first means is adjusted;

a stabilizing network connected to the output of said second operationalamplifier, said stabilizing network providing a leading phase angle tocompensate for the phase lag of said fluid detector and of the fluidconducting paths; and

a valve control switch connected to said stabilizing network forconnecting a source of power to said electromechanical means attached tosaid selector means whereby said selector means is directed to connectsaid second means output to said selector means output when the fluidreceived in said reservoir means from said first means exceeds apredetermined concentration.

1 l. The fluid combining apparatus described in claim wherein saidelectromechanical means is a solenoid and said selector means is asolenoid operated threeway valve.

12. The fluid combining apparatus described in claim 11 wherein saidmixing means comprises:

zle and surrounding the opening of said nozzle in the interior of saidreservoir means, said cylinder having openings in the circumferencethereof adjacent the opening of said nozzle whereby the flow of fluidout of the opening of said nozzle forces fluids from the interior ofsaid reservoir through said openings in said cylinder into the interiorof said cylinder and into the fluid stream issuing from the opening insaid nozzle whereby the mixing of the fluids entering from said nozzleare uniformly and thoroughly mixed with the fluids contained in saidreservoir means.

13. The fluid combining apparatus described in claim 12 wherein saidmeans for producing constant fluid 5 flow comprises a reducing valve.

14. The fluid combining apparatus described in claim 13 furtherincluding a manual selector valve connected between said reducing valveand said detector for selecting one of a plurality of inputs foranalysis by said fluid detector.

15. The fluid conducting apparatus as described in claim 14 wherein saidstabilizing network comprises:

a capacitor;

a first resistor connected and parallel with said capacitor, onejunction of said parallel combination forming the input to saidstabilizing network and the other junction of said first resistor andcapacitor being the output of said stabilizing network; and

a second resistor connected from the output of said stabilizing networkto circuit ground.

1. Fluid combining apparatus comprising: first and second means, eachhaving an input and an output for conducting fluid in one direction fromsaid input to said output, means for maintaining the fluid at saidoutput at substantially constant pressure, said inputs being adapted forconnection to sources of fluid; shunt means connected between theoutputs of said first and second means, said shunt means beingresponsive to the pressure of the fluid at said first means input forconnecting together the outputs of said first and second means when thepressure of the fluid at said first means input falls below apredetermined level; and selector means having two inputs and an outputfor directing fluids from any one of said two inputs of said selectormeans to said output of said selector means, said two inputs beingconnected respectively to the outputs of said first and second means,said selector means being responsive to the pressure of the fluid atsaid second means input for connecting said output of said first meansto said selector means output when the pressure of the fluid at saidsecond means input falls below a predetermined level.
 2. The fluidcombining apparatus described in claim 1 further comprising reservoirmeans having an input and at least one output, said input beingconnected to said selector means output for receiving and combining thefluids conducted by said selector means.
 3. The fluid combiningapparatus described in claim 2 further comprising mixing means withinsaid reservoir means for substantially intermingling and dispersing thefluids uniformly throughout the confines of said reservoir means.
 4. Thefluid combining apparatus described in claim 3 further comprising meansfor producing substantially constant fluid flow connected to said atleast one output of said reservoir means.
 5. The fluid combiningapparatus described in claim 4 further comprising: detector meansconnected to said means for producing constant fluid flow, said detectormeans generating an electrical signal in response to the amount offluids received from said first means in said reservoir means; andelectromechanical means attached to said selector means and saiddetector means, said electromechanical means being responsive to saiddetector means electrical signal for directing said selector means toconnect said second means output to said selector means output wherebythe fluid conducted by said second means is conducted through saidselector means into said reservoir means when the fluids received insaid reservoir means from said first means exceeds a predeterminedconcentration.
 6. The fluid combining apparatus described in claim 5wherein said detector means comprises: a fluid detector having a pair ofelectrical leads between which there is a resistance inverselyproportional to the concentration of the fluids received in saidreservoir means from said first means; and an electronic amplifiercircuit connected to said pair of electrical leads, said amplifierhaving an output connected to said electromechanical means at whichoutput said electrical signal is generated when said fluid detectorresistance falls below a preselected magnitude, said amplifier havingmeans for adjusting said preselected magnitude whereby the fluid mixtureis controlled.
 7. The fluid combining apparatus described in claim 6wherein the fluids are gases and wherein said fluid detector comprises:a polarographic cell having electrolyte in which an anode and cathodeare immersed, and a selective permeable barrier for separating saidelectrolyte from the gases being analyzed, said barrier being permeableto a specific gas whereby the gas enters the electrolyte and reactstherewith to change the resistance between said anode and cathode. 8.The fluid detecting apparatus in claim 7 wherein said specific gas isoxygen.
 9. The fluid combining apparatus described in claim 6 furthercomprising a visual and audible alarm circuit connected to saidelectronic amplifier circuit, said visual and audible alarm circuitbeing activated to produce a visual and audible alarm when theconcentration of the fluid received in said reservoir means from saidfirst means falls outside of a predetermined interval.
 10. The fluidcombining apparatus described in claim 6 wherein said electronicamplifier comprises: a bias voltage supply having its positive terminalconnected to one of said pair of electrical leads of said fluiddetector; a first operational amplifier having its inverting inputconnected to the other of said pair of electrical leads of said fluiddetector; a volt meter connected between the output of said firstoperational amplifier and circuit ground for indicating theconcentration of the fluid received in said reservoir means from saidfirst means; a second operational amplifier having its inverting inputcoupled to the output of said first operational amplifier, said secondoperational amplifier having its non-inverting input connected to anadjustable source of reference voltage whereby the concentration of thefluid received in said reservoir means from said first means isadjusted; a stabilizing network connected to the output of said secondoperational amplifier, said stabilizing network providing a leadingphase angle to compensate for the phase lag of said fluid detector andof the fluid conducting paths; and a valve control switch connected tosaid stabilizing network for connecting a source of power to saidelectromechanical means attached to said selector means whereby saidselector means is directed to connect said second means output to saidselector means output when the fluid received in said reservoir meansfrom said first means exceeds a predetermined concentration.
 11. Thefluid combining apparatus described in claim 10 wherein saidelectromechanical means is a solenoid and said selector means is asolenoid operated three-way valve.
 12. The fluid combining apparatusdescribed in claim 11 wherein said mixing means comprises: a nozzleconnected to the output of said selector three-way valve, said nozzlebeing mounted in the input to said reservoir means; and a cylindricalsection of tubing coaxial with said nozzle and surrounding the openingof said nozzle in the interior of said reservoir means, said cylinderhaving openings in the circumference thereof adjacent the opening ofsaid nozzle whereby the flow of fluid out of the opening of said nozzleforces fluids from the interior of said reservoir through said openingsin said cylinder into the interior of said cylinder and into the fluidstream issuing from the opening in said nozzle whereby the mixing of thefluids entering from said nozzle are uniformly and thoroughly mixed withthe fluids contained in said reservoir means.
 13. The fluid combiningapparatus described in claim 12 wherein said means for producingconstant fluid flow comprises a reducing valve.
 14. The fluid combiningapparatus described in claim 13 further including a manual selectorvalve connected between said reducing valve and said detector forselecting one of a plurality of inputs for analysis by said fluiddetector.
 15. The fluid conducting apparatus as described in claim 14wherein said stabilizing network comprises: a capacitor; a firstresistor connected and parallel with said capacitor, one junction ofsaid parallel combination forming the input to said stabilizing networkand the other junction of said first resistor and capacitor being theoutput of Said stabilizing network; and a second resistor connected fromthe output of said stabilizing network to circuit ground.